Contact

Jens Hoffmann

Address: Spemannstr. 41
72076 Tübingen
Fax: +49 7071 601 702
E-Mail: jens.hoffmann

 

Picture of Hoffmann, Jens

Jens Hoffmann

Position: PhD Student  Unit: Alumni Scheffler

Radiofrequency Transmission Techniques for Human Brain Imaging at 9.4 Tesla


The Max Planck Institute for Biological Cybernetics is equipped with one of the strongest magnets (9.4 Tesla) for human Magnetic Resonance Imaging worldwide. These high-field magnets facilitate structural and functional neuroimaging at unprecedented spatial and temporal resolutions.

 

However, the high field strength comes at the cost of a short wavelength of the radiofrequency (RF) field that is generated by the RF coils. As a result, interference effects cause severe inhomogeneities in the RF field across the human brain. This has two major implications: First, inhomogeneities in the magnetic field component (B1+) lead to an impaired image quality if conventional RF coils and pulses are used. Second, the non-uniform electric field across the brain can cause localized energy dissipation and tissue heating, respectively, and hence poses a potential risk to the subject.

 

This project aims at the development of RF transmission techniques and hardware in order to overcome those problems. In particular, I work on:

 

  • Development of multichannel traveling-wave antennas that serve as simple, remotely-placed RF coils capable of imaging the entire head at 9.4 Tesla.
  • RF shimming: The RF fields emitted by an array of independent coils are superimposed in a way that a more uniform B1+ field in user-defined regions in the brain is achieved.
  • Dynamic parallel transmission (pTx): The single RF fields emitted by a coil array are modulated dynamically in concert with the spatial encoding gradients to actively control the tissue magnetization during an RF pulse and to achieve a more homogeneous excitation.
  • Safety evaluation of self-built RF coils, particularly related to potential tissue heating caused by the RF field during conventional transmission, RF shimming and pTx.

 

All subprojects involve the precise numerical simulation of the RF coils and their electromagnetic field distribution for the purpose of coil design, prototyping and safety evaluation.

Simulation and safety evaluation of self-developed RF coils


Any self-built RF coil must meet requirements for safe application on humans with respect to mechanical and electrical safety and it must be ensured that RF transmission is in compliance with specific absorption rate (SAR) limits in order not to raise tissue temperature above critical levels.

In this project, we developed protocols for safety testing and operational procedures including mechanical and electrical safety, SAR, risk analysis, coil documentation, quality assurance and user training. The local SAR distribution is assessed using numerical simulations that include a precise representation of the coil setup, the scanner environment and the input- and decoupling circuits (Fig. 1A). Simulations are thoroughly validated via comparison to experiment (Fig. 1B) before SAR is assessed in human voxel models. Moreover, these simulations support the design of novel coils and provide insight into RF field propagation phenomena at ultra-high fields.

 

Figure 1

 

RF Shimming


RF shimming Figure 1 or static parallel transmission uses multiple independent RF coils to control the RF field pattern across the human body. In order to improve the B1+ pattern in a pre-defined body region, for example to achieve a more homogeneous B1+ field, a fixed set of RF voltages with optimized amplitudes and phases is calculated and applied to the single coil elements. An optimized excitation mode can readily be used with any standard MR sequence in order to improve image quality.

In this subproject, we developed techniques for rapid, subject-specific single-channel B1+ mapping and for the numerical calculation of optimized RF shim configurations. The performance and limitations of RF shimming at 9.4 T using a state-of-the-art dual-row transmit array were investigated with respect to B1+ homogeneity, efficiency and SAR.

 

Multimode traveling-wave imaging


At a frequency of 400 MHz, the bore of a human 9.4 Tesla scanner acts as a waveguide that conveys the RF field from a remotely placed antenna towards the subject. The propagation of the RF field can be interpreted as a superposition of three basic waveguide modes.

In this project, we designed and evaluated a simple three-port antenna (Fig. 3A) that can be positioned distant to the subject and that is capable of controlling all three waveguide modes simultaneously (Fig. 3B). Two time-interleaved acquisitions using complementary RF shims (TIAMO) and their subsequent combination facilitate 9.4 T images of the entire brain without signal dropouts (Fig. 3C). This makes the antenna suitable as a remote 1H volume coil for anatomical localization and B0 shimming. The compatibility of similar designs with dedicated receive arrays for 1H, 31P or 23Na applications was demonstrated.

 

Figure 3

 

Dynamic parallel transmission


The dynamic parallel transmission (pTx) method persistently and independently varies the RF amplitudes and phases on multiple coil elements using dedicated hardware and hence dynamically changes the B1+ pattern during the RF pulse. This is performed in concert with the spatial encoding gradients in order to directly control the buildup of the magnetization, for example to achieve a homogeneous flip angle (Fig. 4A) or a desired flip angle distribution (Fig. 4B) in a selected region in the human brain.

This subproject aims at the improvement of different aspects of pTx methodology to take the method towards routine application at 9.4 T. It involves the development of optimized coil arrays and associated hardware, comprehensive numerical simulations to ensure compliance with SAR limits as well as rapid volumetric B1+ and B0 calibration scans that constitute the basis for subsequent, subject-specific pTx pulse design.

 

Figure 4

Since 05/2009

PhD student in the Graduate School of Neural & Behavioural Sciences / International Max Planck Research School at the High-Field Magnetic Resonance Center, MPI for Biological Cybernetics. PhD project: "RF Transmission Techniques for Human Brain Imaging at 9.4 Tesla".

2008/2009

Diploma Thesis „Simulation of Electromagnetic Fields for the Development of NMR Coils“ at the High-Field Magnetic Resonance Center, MPI for Biological Cybernetics, Tübingen, Germany. Thesis supervisors: Dr. Rolf Pohmann, Prof. Dr. Wilhelm Kley.

2007

Student research project in scientific computing at the Faculty of Mathematics, University of Tübingen, Germany: "Numerical Integration of the Time-Dependent Schrödinger Equation through Fourier Basis and Strang-Splitting “

10/2002 – 05/2009

Diploma in Physics with focus on scientific computing at the University of Tübingen, Germany

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Articles (14):

Avdievich NI, Hoffmann J, Shajan G, Pfrommer A, Giapitzakis IA, Scheffler K and Henning A (February-2017) Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T NMR in Biomedicine 30(2) 1-12.
Hoffmann J, Henning A, Giapitzakis IA, Scheffler K, Shajan G, Pohmann R and Avdievich NI (September-2016) Safety testing and operational procedures for self-developed radiofrequency coils NMR in Biomedicine 29(9) 1131–1144.
Tse DH, Wiggins CJ, Ivanov D, Brenner D, Hoffmann J, Mirkes C, Shajan G, Scheffler K, Uludağ K and Poser BA (June-2016) Volumetric imaging with homogenised excitation and static field at 9.4 T Magnetic Resonance Materials in Physics, Biology and Medicine 29(3) 333–345.
Mirkes C, Shajan G, Bause J, Buckenmaier K, Hoffmann J and Scheffler K (March-2016) Triple-quantum-filtered sodium imaging at 9.4 Tesla Magnetic Resonance in Medicine 75(3) 1278–1289.
Shajan G, Mirkes C, Buckenmaier K, Hoffmann J, Pohmann R and Scheffler K (February-2016) Three-layered radio frequency coil arrangement for sodium MRI of the human brain at 9.4 Tesla Magnetic Resonance in Medicine 75(2) 906–916.
Hoffmann J, Mirkes C, Shajan G, Scheffler K and Pohmann R (January-2016) Combination of a multimode antenna and TIAMO for traveling-wave imaging at 9.4 Tesla Magnetic Resonance in Medicine 75(1) 452–462.
Mirkes CC, Hoffmann J, Shajan G, Pohmann R and Scheffler K (January-2015) High-resolution quantitative sodium imaging at 9.4 tesla Magnetic Resonance in Medicine 73(1) 342–351.
Hoffmann J, Shajan G, Scheffler K and Pohmann R (October-2014) Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array Magnetic Resonance Materials in Physics, Biology and Medicine 27(5) 373-386.
Shajan G, Kozlov M, Hoffmann J, Turner R, Scheffler K and Pohmann R (February-2014) A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T Magnetic Resonance in Medicine 71(2) 870–879.
Hoffmann J, Shajan G, Budde J, Scheffler K and Pohmann R (May-2013) Human brain imaging at 9.4 T using a tunable patch antenna for transmission Magnetic Resonance in Medicine 69(5) 1494–1500.
Pohmann R, Shajan G, Hoffmann J, Budde J, Hagberg G, Bieri O, Bisdas S, Ernemann U, Weigel M, Ehses P, Hennig J, Chadzynski G and Scheffler K (April-2013) Imaging and Spectroscopy at 9.4 Tesla: First Results on Patients and Volunteers MAGNETOM Flash 2013(2) 58-67.
Shajan G, Hoffmann J, Balla DZ, Deelchand DK, Scheffler K and Pohmann R (October-2012) Rat brain MRI at 16.4T using a capacitively tunable patch antenna in combination with a receive array NMR in Biomedicine 25(10) 1170–1176.
Shajan G, Hoffmann J, Budde J, Adriany G, Ugurbil K and Pohmann R (August-2011) Design and Evaluation of an RF Front-End for 9.4 T Human MRI Magnetic Resonance in Medicine 66(2) 594–602.
Budde J, Shajan G, Hoffmann J, Ugurbil K and Pohmann R (February-2011) Human imaging at 9.4 T using T2*-, phase-, and susceptibility-weighted contrast Magnetic Resonance in Medicine 65(2) 544-550.

Conference papers (1):

Mirkes C, Shajan G, Hoffmann J, Bause J, Buckenmaier K and Scheffler K (November-2015) Natrium MRT bei 9,4 Tesla, 18. Jahrestagung der Deutschen Sektion der ISMRM e.V. (DS ISMRM 2015), 23-27.

Posters (19):

Shajan G, Hoffmann J, Adriany G, Ugurbil K and Scheffler K (May-11-2016): A 7T head coil with 16-channel dual-row transmit and 32-channel receive array for pTx applications and high SNR, 24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016), Singapore.
Mirkes C, Shajan G, Bause J, Buckenmaier K, Hoffmann J and Scheffler K (June-4-2015): Triple-quantum-filtered sodium imaging at 9.4 Tesla, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada.
Shajan G, Hoffmann J, Scheffler K and Pohmann R (June-2-2015): An interface to connect a 16-channel transmit array to an 8-channel parallel transmit system, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada.
Shao T, Avdievich NI, Chang P, Hoffmann J, Scheffler K and Henning A (June-1-2015): Systematic investigation of influence factor on parallel transmit pulse performance at 9.4 Tesla, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada.
Hoffmann J, Shajan G, Mirkes C, Shao T, Henning A, Pohmann R and Scheffler K (June-1-2015): Towards routine application of dynamic parallel transmission for whole-brain imaging at 9.4 Tesla, 23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015), Toronto, Canada.
Chadzynski GL, Henning A, Ehses P, Hoffmann J, Shajan G and Scheffler K (May-15-2014): High-Resolution Free Induction Decay Proton MRSI in the Human Brain at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Mirkes CC, Shajan G, Hoffmann J, Brenner D, Pohmann R and Scheffler K (May-15-2014): Sodium B1 mapping at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Goense J, Beyerlein M, Hoffmann J, Shajan G, Steudel T, Scheffler K, Logothetis NK and Merkle H (May-12-2014): Novel RF-Coil Assembly to Simultaneously Investigate fMRI and Electrophysiology in Non-Human Primates in a Large Bore Vertical Magnet, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy.
Bakhtiary M, Shajan G, Hoffmann J, Scheffler K and Pohmann R (April-25-2013): 31P-spectroscopy on humans at 9.4 T in combination with a patch antenna for proton imaging: Initial results, 21st Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2013), Salt Lake City, UT, USA.
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Mirkes C, Hoffmann J, Shajan G, Pohmann R and Scheffler K (April-25-2013): Combination of a sodium birdcage coil with a tunable patch antenna for B0 shimming and anatomical localization at 9.4 T, 21st Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2013), Salt Lake City, UT, USA.
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Mirkes C, Hoffmann J, Shajan G, Pohmann R and Scheffler K (April-23-2013): Amide Proton Chemical Exchange Saturation Transfer at 9.4 T with Optimized RF Transmit Field Through B1 Shimming, 21st Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2013), Salt Lake City, UT, USA.
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Bause J, Shajan G, Hoffmann J, Scheffler K and Pohmann R (May-7-2012): Design and SAR Estimation of a Segmented Loop for Dual Coil CASL at 9.4 T, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia.
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Shajan G, Hoffmann J and Pohmann R (May-12-2011): A 15-Channel Receive Array & 16-Channel Detunable Transmit Coil for Human Brain Imaging at 9.4T, 19th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2011), Montréal, Canada.
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Deistung A, Budde J, Schweser F, Hoffmann J, Pohmann R and Reichenbach JR (May-10-2011): High Resolution Quantitative Susceptibility Mapping at 9.4T, 19th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2011), Montréal, Canada.
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Shajan G, Hoffmann J, Balla DZ and Pohmann R (May-6-2010): A 700MHz Receive Array using Patch Antenna for Spin Excitation, ISMRM-ESMRMB Joint Annual Meeting 2010, Stockholm, Sweden.
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Hoffmann J, Shajan G and Pohmann R (May-6-2010): Capacitively Tunable Patch Antenna for Human Head Imaging at 9.4 Tesla, ISMRM-ESMRMB Joint Annual Meeting 2010, Stockholm, Sweden.
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Hoffmann J, Budde J, Shajan G and Pohmann R (May-6-2010): Slice-Selective B1 Phase Shimming at 9.4 Tesla, ISMRM-ESMRMB Joint Annual Meeting 2010, Stockholm, Sweden.
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Budde J, Shajan G, Mühlbauer F, Hoffmann J, Ugurbil K and Pohmann R (May-4-2010): Human T2* and Phase Imaging at 9.4 T, ISMRM-ESMRMB Joint Annual Meeting 2010, Stockholm, Sweden.
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Shajan G, Balla DZ, Hoffmann J and Pohmann R (October-2008): A Microstrip Resonator for Animal MRI at 16.4 T, ESMRMB 2008 Congress: 25th Annual Meeting, Valencia, Spain, Magnetic Resonance Materials in Physics, Biology and Medicine, 21(Supplement 1) 449-450.

Theses (1):

Hoffmann J: Simulation of Electromagnetic Fields for the Development of NMR Coils, Eberhard-Karls-Universität Tübingen, Germany, (January-2009). Diplom thesis
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Popular Scientific Articles (1):

Buckenmaier K, Shajan G, Chadzynski G, Hoffmann J, Pohmann R and Scheffler K: Magnetresonanztomografie bei ultrahohen Feldstärken - Forschungsbericht 2014 Max-Planck-Institut für biologische Kybernetik, Jahrbuch der Max-Planck-Gesellschaft 2014.

Talks (10):

Shajan G, Mirkes C, Hoffmann J, Scheffler K and Pohmann R (May-14-2014) Abstract Talk: A Three-Layered Coil Arrangement for Sodium Imaging of the Human Brain at 9.4T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy(0620).
Avdievich NI, Pfrommer A, Hoffmann J, Chadzynski GL, Scheffler K and Henning A (May-14-2014) Abstract Talk: Tranceive Phased Array with High Transmit Performance for Human Brain Application at 9.4 T, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy(0622).
Hoffmann J, Mirkes C, Shajan G, Scheffler K and Pohmann R (May-13-2014) Abstract Talk: A 3-Port Traveling-Wave Antenna in Combination with TIAMO for the Acquisition of Void-Free Brain Images at 9.4 Tesla, Joint Annual Meeting ISMRM-ESMRMB 2014, Milano, Italy(0399).
Mirkes C, Hoffmann J, Shajan G, Pohmann R and Scheffler K (October-5-2013) Abstract Talk: Quantitative Sodium Imaging at 9.4 T, 30th Annual Scientific Meeting ESMRMB 2013, Toulouse, France, Magnetic Resonance Materials in Physics, Biology and Medicine, 26(Supplement 1) 370.
Hoffmann J, Shajan G, Scheffler K and Pohmann R (October-4-2013) Abstract Talk: Combination of Multimode Traveling-Wave Excitation and TIAMO for Human Brain Imaging at 9.4 T, 30th Annual Scientific Meeting ESMRMB 2013, Toulouse, France, Magnetic Resonance Materials in Physics, Biology and Medicine, 26(Supplement 1) 192.
Mirkes CC, Shajan G, Hoffmann J and Scheffler K (May-2013) Abstract Talk: Amide Proton Chemical Exchange Saturation Transfer at 9.4T, 3rd International Workshop on Chemical Exchange Saturation Transfer Imaging (OctoberCEST 2012), Annapolis, MD, USA, Contrast Media & Molecular Imaging, 8(3) 311-312.
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Hoffmann J, Shajan G, Scheffler K and Pohmann R (April-23-2013) Abstract Talk: RF Shimming Capabilities at 9.4 Tesla using a 16-channel Dual-Row Array, 21st Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2013), Salt Lake City, UT, USA 901.
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Shajan G, Hoffmann J, Scheffler K and Pohmann R (October-2012) Abstract Talk: A 31-Element Receive Array for Human Brain Imaging at 9.4T, 29th Annual Scientific Meeting ESMRMB 2012, Lisboa, Portugal, Magnetic Resonance Materials in Physics, Biology and Medicine, 25(Supplement 1) 265-266.
Shajan G, Hoffmann J, Scheffler K and Pohmann R (May-8-2012) Abstract Talk: A 16-Element Dual-row Transmit Coil Array for 3D RF Shimming at 9.4 T, 20th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2012), Melbourne, Australia(308).
Hoffmann JO, Shajan G and Pohmann R (May-9-2011) Abstract Talk: Human Brain Imaging at 9.4 Tesla Using a Combination of Traveling Wave Excitation with a 15-Channel Receive-Only Array, 19th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2011), Montréal, Canada 157.
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Last updated: Monday, 22.05.2017